Atmospheric Dispersion Corrector (ADC)
As of Dec 2010 we have the ADC on offer for regular use. The ADC can
be used to minimise the wavelength-dependent fiber-aperture losses that
are due to the atmospheric dispersion at large zenith distances.
The ADC swings into the telescope beam before the FIES/STANCAM pickoff
mirrors, and provides an unvignetted field of view of about 3
arcminutes. The ADC consists of a set of two rotating prisms,
that together compensate for the atmospheric effect.
The ADC was primarily designed for use with FIES, but can be
also be used with ALFOSC or NOTCAM.
For use with FIES the ADC is expected to be advantageous in terms of
throughput and radial-velocity precision for sources at airmass>1.5
(or so). Detailed information about throughput etc. can be found in
the commissioning document.
Note: in the K-band significant emission from the ADC prisms should be expected,
leading to about 0.5 mag higher background level.
|ADC transmission as observed
by ALFOSC and NOTCAM|
| U || B || V || R || I
|| J || H || K |
| 88% || 88% || 90% || 91% || 8
9% || 84% || 85% || 67% |
Commands to operate the ADC
The ADC can be controlled from the TCS user-interface, and more commonly through
sequencer commands on the instrument control computer (marissa, selena, verushka).
The below table lists the most useful sequencer commands.
A complete list of ADC
sequencer commands can be found here.
The full explanation of the commands is given in the main TCS
command reference .
|Some useful ADC Sequencer commands|
| adc-arm-in |
| adc-arm-out |
| adc-mode-automatic-when-presetting |
| adc-mode-automatic-off |
| adc-actual |
| adc-explicit |
| wait-adc |
Note that the continuously updating ADC mode
(adc-mode-automatic-continous) should be used with caution, as this
mode will displace the star(s) during an exposure. The continuous ADC mode can only be used
in combination with the FIES fiber guider.
In general, we advise to use the automatic mode
adc-mode-automatic-when-presetting which updates the ADC prisms
every time the telescope is pointed to a new target.
Telescope focus offset
When the ADC is in the beam the telescope focus should be offset by
about 560 units. This offset has to be applied by the user (with
focus-position, or focus-delta).
When using the ADC with ALFOSC, the offset will correctly be taken
into account when using the command alfosc.focus-offset .
Use with FIES
See the FIES
Use with ALFOSC
Note that the Field-Of-View of the ADC is only about 3 arcminutes!
To use the ADC, issue the following commands before pointing
to a new target:
and then the ADC will then adjust itself while presetting to new targets.
To have the ADC adjust itself to the current telescope altitude, e.g.
when following the same target for an extended period:
but never do that during an exposure, as the target will move somewhat
with respect to the fiber.
To stop using the ADC issue the following commands: